In the past, watermarks or a shadow marks have been used to create transparency images in paper. Typically, watermarks and shadow marks are produced by inducing localized variations in the thickness of the cellulosic web. This variation in thickness, in turn, creates localized variations in the opacity of the paper, and so creates a contrast which makes the watermark visible, particularly in transmitted light. The desired localized variation in web thickness is typically effected by fiber displacement by means of a dandy roll which runs on top of the wet web on the wire of a Fourdrinier paper machine. The dandy roll imprints the desired image into the wet web creating variations in thickness that define a watermark or shadow mark.
However, the image imprinted by the dandy rolls cannot be easily changed. The dandy roll has protrusions (to form watermarks) or depressions (to form shadow marks) that create the variation in thickness during the web formation process (e.g., while the web is still wet) resulting in the variation in transparency. Changing these protrusions or depressions in the dandy roll is not practical in most instances. Thus, in order to form a different image, a new dandy roll is required. However, the production of dandy rolls is typically an expensive process.
Another method can be used to produce transparency images with a printer such as a Flexographic printer or a screen printer. This method, sometimes called false watermarking, is done by printing a paper web with inks containing materials such as oils, waxes or glycerin which penetrate the paper but which have little or no colorant. The printed areas are less opaque due to replacement of some fiber/air interfaces with fiber/ink interfaces. Although this method does produce transparency images, it is not easily adaptable to customized production, since the time and expense required to prepare a screen or flexo plate cannot be justified economically unless a large number of prints are produced.
As such, a need currently exists for a more efficient and versatile method of making a fibrous substrate (e.g., paper) that has variations in the opacity of the substrate, while allowing the patterns or designs to vary as desired. Ideally, the method will make use of digital printing, which is readily adaptable and versatile, enabling production of prints without the need for plates or screens.